In June 2011, after the sixth game of the Stanley Cup Finals between the Vancouver Canucks and the Boston Bruins, in Boston, the series was locked in a tie. The Canucks, based in the Pacific Time Zone, had lost all the games hosted in Boston. The Bruins, located in the Eastern Time Zone, had fallen in every contest hosted in Vancouver. As the team with the better regular-season record, the Canucks held home-ice advantage for the seventh and deciding game. So when the Bruins arrived in Vancouver the day before that matchup, they went searching for an edge.

“I was getting ready to take the stage for a lecture in Minnesota when my phone rings,” says Charles A. Czeisler, Ph.D., M.D., sitting in a small, windowless conference room outside his office at the Brigham and Women’s Hospital at Harvard Medical School in Boston, where he serves as the chief of the Division of Sleep Medicine. “It’s the Bruins’ team physician, who says, ‘I’m here in Vancouver, and I’m wondering if you have any suggestions for what we might do.’”

Czeisler asked a number of questions about the Bruins’ travel schedule. He discovered that the team planned to take their discipline of napping in the afternoon with them to Vancouver the next day. “I told him that that doesn’t work—the team needs to be napping in the morning in Vancouver, because that is afternoon here in Boston.” In other words, to maximize their energy and mood, the players should keep their bodies on Eastern Time. “There is a circadian rhythm to athletic performance,” he says, referring to the study of the human body’s inner clockwork.

Physiologically, the window for peak focus, strength, reaction time, and physical flexibility arrives in late afternoon or early evening, when “the body is sending out its strongest drive for wakefulness,” says Czeisler. According to him, that’s when most Olympic records are broken. Since the opening face-off was scheduled for 5 p.m. Vancouver time—8 p.m. in Boston—Czeisler understood that if the Bruins napped at just the right time, the game would fall into a wakeful sweet spot.

The team canceled its morning practice the next day. “All the sports-talk guys are freaking out—‘They’re not doing the shootaround before the championship game! What are they thinking?’” he says. “I was pleased to see that they won the game, which was pretty cool.”

In fact, the Bruins shut out the Canucks 4–0. Whether or not the three-hour shift in the team’s nap time played a role, it is increasingly common in today’s competitive sports to encounter teams at both the professional and collegiate levels who are working to manipulate sleep to their advantage. Northwestern University head football coach Pat Fitzgerald instituted team-wide naps after arriving, and last season he imposed sleep-monitoring sensors on his players during the season. In professional baseball, East Coast teams playing home games against West Coast-based teams have a measurable advantage. “They win about 5% more games,” says Czeisler. “And on average, they score about a quarter more runs.” In 2013, a colleague of Czeisler’s at Harvard published the findings of a study in the journal Sleep, arguing that East Coast-based teams in the NFL consistently underperform when competing in away games on the West Coast.

“It’s one thing to see regular people traveling and changing time zones and, say, visiting the Louvre, and not noticing a split-second change in reaction time when looking at a painting,” says Czeisler, who notes that the brain’s normal reaction time to a stimulus is a quarter of a second—which can quadruple or more if the person is severely sleep deprived. “But for an elite athlete, sleep deprivation degrades coordination and the ability to learn and consolidate memories. Your emotions are more volatile, too. Not getting enough sleep will degrade athletic performance.”

Czeisler isn’t the only sleep expert moonlighting in athletics, but he is the only one whose prominence in professional sports has earned him the unofficial title, at least in the NBA, as “the Sleep Doctor.”